A police officer stands guard outside Leytonstone station in north London on Dec. 6, 2015, a day after three people were stabbed in what police are treating as a “terrorist incident.”

Photo by Leon Neal/AFP/Getty Images

Following a stabbing attack at a London subway station that police are investigating as a “terrorist incident,” Britons on social media united behind one phrase uttered by a bystander. Video posted on social media shows police pinning down the attacker as an onlooker yells: “You ain’t no muslim, bruv.” That quickly became the rallying cry after the attack with many describing it as the perfect way for Londoners to respond and express their defiance. The attacker injured three people, one of them seriously, at the Leytonstone subway station before reportedly saying, “This is for Syria.”